1. Field of the Invention
The present invention relates to a linear motor, a movable stage and an electron microscope.
2. Description of the Related Art
In an electron microscope, it is common to move an imaging object using a movable stage and to image the imaging object at a variety of positions. Because high speed move and highly precise positioning of the movable stage are required, a linear motor is used as a drive source for the movable stage. However, in the electron microscope, magnetic field variation on an electron orbit should be reduced. Therefore, magnetic field variation caused by movement of the movable stage should also be reduced.
In the linear motor, the driving force is an electromagnetic force generated between a coil energized by control current and permanent magnets. A moving coil type that is common in the liner motor includes a stator having a row of permanent magnets arrayed in a rail shape and a movable element having coils energized by the control current. The length of the stator in the moving direction of the movable element is generally equal to the moving stroke of the movable element.
When a linear motor is used for a movable stage of an electron microscope, if the stator is fixed, magnetic field variation can be suppressed. However, as a movable stage with less number of parts, such a configuration of moving the stator is also possible. In the configuration, because magnetic field variation on an electron orbit occurs due to movement of the stator, the magnetic field variation should be suppressed. Also, in order to increase the driving performance, weight reduction of the stator is also required.
In Japanese Unexamined Patent Application Publication No. 2002-354779 for example, a linear motor is disclosed in which an end of a yoke is folded in an L-shape toward the surface of an armature coil so as to cover an end of permanent magnets. With the configuration, leakage flux from a motor can be reduced.
However, in the linear motor described in Japanese Unexamined Patent Application Publication No. 2002-354779, no consideration is given to that there is an individual variation on the magnitude of a leakage magnetic field of a stator. That is, because a same stator yoke is used although acceptable amount of the leakage magnetic field is different in each device, there is a problem that the weight of the stator becomes heavy unnecessarily.
An object of the present invention is to balance suppression of the leakage magnetic field and the driving performance.